Holograms become more and more popular for a plurality of applications, such as liquid crystal displays or backlight units of e.g. vehicles. For recording a hologram having a desired grating, a recording medium can be irradiated by a laser light beam (e.g.: H. M. Smith in “Principles of Holography”, Wiley-Interscience 1969). One known method is the so called dual-beam method. In this method, a first laser light beam and a second laser light beam are provided by preferably a single laser light source. By irradiating the recording medium with both laser light beams, a static intensity modulation is generated at the location of the recording medium due to optical interference. According to the applied intensity modulation, a refractive index modulation is formed through a photo reaction. Within the generated grating or modulation, the holographic information is stored.
In an exemplified application of an illumination system, such as a directional backlight unit for general electronics, automotive electronic displays, flat illumination devices for general lighting, illuminated signage applications, a hologram produced according to the above described method is usually illuminated by one or more light sources, such as lasers or LEDs (light-emitting diodes). For instance, a plurality of LEDs can be positioned at a narrow side or edge face of a light guiding stack comprising at least the recorded medium.
A top view of an exemplified embodiment of an essentially plane illumination system 26′ according to the prior art having a medium 6.1′ which has been recorded according to a method of the prior art is depicted in FIG. 6a. In particular, the depicted embodiment shows the light picture emitted by the illumination system 26′ if the viewing angle of an observer is 90° to the plane of the illumination system 26′. In the shown embodiment, three light sources 20.1 to 20.3, such as LEDs, are arranged at the (left) narrow side 28′ of the light guiding stack 32′ having a recorded medium 6.1′. Each light source 20.1 to 20.3 generates a light beam which propagates within the recorded medium 6.1′ due to total internal reflection. The light is coupled out by suitable coupling out structures (not shown). As can be seen from FIG. 6a, the three light sources 20.1 to 20.3 generate three essentially parallel light strips 22.1 to 22.3. A disadvantage of the recorded medium 6.1′ of prior art is that no homogenous light emitting surface about essentially the total area of the broad surface of the light guiding stack 32′ is perceived by an observer. For instance, between light strip 22.1 and light strip 22.2 as well as between light strip 22.2 and light strip 22.3, an unlighted strip can be perceived.
The FIG. 6b shows another view of the same example as shown in FIG. 6a. The difference between FIGS. 6a and 6b is the viewing angle of the observer. The viewing angle in FIG. 6b is 60°. As can be seen from FIG. 6b, in comparison to the course of the light strips in FIG. 6a, the different viewing angle causes that the visible light strips are perceived at a different angle. However, also in this case (and every other viewing angle), no homogenous light emitting surface about the (essentially) total surface of the light guiding stack 32′ can be perceived.